Secure labeling or tagging of items is important for tracking of the items and prevention of counterfeiting. Some uses of secure tagging include identifying, authenticating, tracking, or validating documents, pharmaceuticals, consumer goods, and high value components and encoding of information on such items.
One technique for tagging items is to encode information using luminescent taggants. Semiconductor nanocrystals, for example, can be tailored to fluoresce and emit a narrow band of light centered at a tuned frequency. The size, shape, and materials chosen for a nanocrystal generally determines the emission properties of the nanocrystal and the frequencies emitted. Examples of size-tunable nanocrystals with narrow tunable spectra include particles of II-VI semiconductor materials such as CdSe, CdS, and CdTe and particles of III-IV semiconductor materials such as InP, GaP, and GaN. Composite particles consisting of, for example, wide bandgap shells such as ZnS surrounding smaller bandgap cores, such as CdSe, are also frequently employed. FIG. 1 shows plots 110 to 150 of emission spectra of CdSe/ZnS core-shell particles with a range of diameters between about 2 nanometers and 5 nanometers to illustrate how the proper selection of the size of the nanocrystals can provide a desired narrow-band emission.
Narrow-band taggants can be placed in marks that encode information in the form of the emission spectra and the spatial locations of the marks. For example, various combinations of nanocrystals can be placed in a series of locations or marks on an object to encode information. A reading device can illuminate the taggants with a wavelength of light that stimulates the emission from the nanocrystals and then spectrally analyze the resulting emissions to decode the information. For example, a particular code sequence could consist of nanocrystals that emit light within a narrow band in the green portion of the spectrum at a first location on an item, blue-emitting nanocrystals at a second location on the item, and red-emitting nanocrystals at a third location on the item, and the green-blue-red combination can be interpreted to have a particular data value. More generally, any number of locations and combinations of nanocrystal types can be used for encoding the data.
Manufactured tagging systems can have complicated emission spectra instead of narrow band spectra. Examples of suitable material for taggants with complex emission spectra include doped garnets such as yttrium-aluminum-garnet (YAG) doped with rare earths such as europium. Alternatively, nanocrystals that have narrow band emission spectra can be combined to produce tagging systems with complex emission signatures. These complex emission spectra can be advantageous in preventing counterfeiting of marks because complicated spectral signatures are generally more difficult to replicate.
Most emission spectra, even complicated spectra, can be duplicated using appropriate combinations of nanocrystals with narrow emission lines. The emissions spectra 110 to 150 for CdSe/ZnS particles, for example, illustrate a set of the narrow-band spectra that can be used as a basis set that when used in different combinations and relative proportions can match a wide range of arbitrary spectra. Changing the excitation wavelength used for a reading process can make counterfeiting more difficult because the emission spectrum of a taggant such as a nanocrystal of a particular type will typically depend upon the wavelength or wavelengths used to stimulate the emission. Thus, if a combination of nanocrystals of another material is chosen that matches the emission spectrum of a mark under illumination by one wavelength, the combination may fail to provide the correct readout spectrum when excited by a different wavelength. However, if the nanocrystals are non-interacting, the differences in emission as a function of stimulating wavelength can often still be duplicated using an appropriate combination of nanocrystals and counterfeiting is often possible.